Can a pointer to base point to an array of derived objects?

Question

I went to a job interview today and was given this interesting question.

Besides the memory leak and the fact there is no virtual dtor, why does this code crash?

#include <iostream>  //besides the obvious mem leak, why does this code crash?  class Shape { public:     virtual void draw() const = 0; };  class Circle : public Shape { public:     virtual void draw() const { }      int radius; };  class Rectangle : public Shape { public:     virtual void draw() const { }      int height;     int width; };  int main() {     Shape * shapes = new Rectangle[10];     for (int i = 0; i < 10; ++i)         shapes[i].draw(); } 

Answer 1

You cannot index like that. You have allocated an array of Rectangles and stored a pointer to the first in shapes. When you do shapes[1] you're dereferencing (shapes + 1). This will not give you a pointer to the next Rectangle, but a pointer to what would be the next Shape in a presumed array of Shape. Of course, this is undefined behaviour. In your case, you're being lucky and getting a crash.

Using a pointer to Rectangle makes the indexing work correctly.

int main() {    Rectangle * shapes = new Rectangle[10];    for (int i = 0; i < 10; ++i) shapes[i].draw(); } 

If you want to have different kinds of Shapes in the array and use them polymorphically you need an array of pointers to Shape.

Answer 2

As Martinho Fernandes said, the indexing is wrong. If you wanted instead to store an array of Shapes, you would have to do so using an array of Shape *'s, like so:

int main() {    Shape ** shapes = new Shape*[10];    for (int i = 0; i < 10; ++i) shapes[i] = new Rectangle;    for (int i = 0; i < 10; ++i) shapes[i]->draw(); } 

Note that you have to do an extra step of initializing the Rectangle, since initializing the array only sets up the pointers, and not the objects themselves.